This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-275402 filed Sep. 11, 2000.
The present invention relates to an antenna for a portable radio communication device and a method of transmitting a radio signal using such an antenna. This antenna and method may be used for radio-transmitting biomedical signals such as human pulse waves or lock/unlock control signals in a vehicle keyless entry system.
Conventional portable radio communication devices use rod antennas or loop antennas for radio signal transmission. If these antennas are built in the portable radio communication devices, the antenna efficiency is limited due to limitation in size of casings of the transmitters.
JP-A-11-163757 proposes to use a metal cover for a built-in battery of a radio communication device and use the metal cover as an antenna. This arrangement enables size reduction of the radio communication device without lessening antenna efficiency. It is however still likely that the antenna efficiency is limited due to limitation of size of the radio communication device, because the battery is built in the radio communication device.
It is therefore an object of the present invention to provide an antenna for a portable radio communication device and a method of transmitting a radio signal with an improved antenna performance.
It is a further object of the present invention to provide an antenna for a portable radio communication device, which is wearable on a human body or holdable by a human body and uses the human body as a part of the antenna for improving an antenna gain.
According to the present invention, an antenna is provided for a portable radio communication device, which has a built-in battery and a transmission circuit in a resin-made casing. The transmitter circuit is connected to a conductive member in the casing. The conductive member may be a cover of the battery or a plate. The conductive member is disposed to be capacitively coupled with a part of a human body through the casing when the casing is attached to the human body, so that an antenna element of an electric field-type antenna is formed to transmit output signals of the transmitter circuit.
Preferably, a magnetic field-type antenna is provided in the casing and combined with the electric field-type antenna to make the directivity of radiation to be isotropic.